Optical storage media has proven to be attractive for the high density storage of information. More recently, optical storage media and systems have been disclosed in which a magneto-optical storage medium can have information written thereon overwritten with different information. Typically, in order to overwrite the material already stored on the magneto-optical medium, the medium must first be returned to an initial state. The magneto-optical medium can be returned to the initial state on the entire storage region or the initialization can take place in a predetermined region of the medium. When the initializing activity occurs immediately prior to and only in the region to be overwritten, the procedure is referred to as a direct overwrite.
In the related art, direct overwrite was accomplished by providing a first magnet and associated radiation (heat) source. The radiation source would heat a local area of the magneto-optical recording layer above an ordering temperature while the associated magnet would impose a predetermined initial state on the recording layer. A separate second magnet and associated radiation source were located such that the motion of the storage media would move the now oriented (i.e., by the first magnet and radiation source) region of the storage layer into position wherein the second magnet, with a magnetic field having an opposite orientation to the magnetic orientation of first magnet, applies a magnetic field thereto. The radiation from the second radiation source is applied to the storage layer region having the magnetic field from the second magnet applied thereto. When the information to be stored requires an orientation of the magneto-optical layer parallel to the orientation provided by the first magnet and first radiation source, then the power of the second radiation source is kept at a low enough value so that the illuminated region remains below the magnetic ordering temperature of the storage layer. When the information to be stored requires an orientation opposite to the orientation provided by the first magnet and the first radiation source, the power of the second radiation source is elevated to provide a temperature for the storage layer above the magnetic ordering temperature of the storage layer. As the region (or part thereof) heated above the ordering temperature is cooled in the presence of the magnetic field from the second magnet, this region of the storage layer will have an orientation opposite to that of orientation provided by the first magnet and the first radiation source. In this manner, information can be stored on storage medium which is determined by the orientation of local regions of a magneto-optical material.
Referring to FIG. 1, the direct write process is illustrated. The magneto-optical storage medium 10 is moving past a first radiation beam 11, a second radiation beam 12, the magnetic field 13A of a first magnet 13, and the magnetic field 14A of a second magnet 14 in a direction indicated by the arrow v. The storage medium 10 is constrained to move such that a magneto-optical material in information track 5 interacts with the first radiation beam 11 and the first magnetic field 13A and subsequently with the second radiation field 12 and the second magnetic field 14A. The magneto-optical material of track 5 has, prior to interacting with the radiation and the magnetic fields, initial information regions 15 which have an orientation opposite to the orientation of the remainder of the magneto-optical material. As a result of the interaction with the first radiation field 11 and the first magnetic field 13A, the information regions (and the remainder of the magneto-optical material of the information track 5) are oriented in an initialized orientation. Therefore, as seen in region 17 of the information track 5, no information regions are present. As the information track 5 moves past the second radiation field 12 and the second magnetic field 4A, the radiation beam 12 is modulated to heat only selected regions above the magneto-optical material ordering temperature. Upon cooling in the presence of magnetic field 14A, the regions 16 which have been heated above the ordering temperature now have an orientation opposite to the orientation of the surrounding initialized magneto-optical material. The regions 16 become the directly overwritten (or updated) information.
The foregoing process, conceptually easy to understand, is difficult to implement in practice. In order to achieve a high density of information, the information regions must be very small. The small size of the information regions means that the positioning of the second radiation beam and the second magnetic field relative to the first radiation beam and first magnetic field is critical. Furthermore, the two sets of magnets and radiation sources would preferably be mounted on the same read/write head, a component of relatively small dimensions. However, the weight of these components would limit the dynamic performance of the read/write head.
A need has therefore been felt for apparatus and an associated method for performing a direct overwrite procedure on a magneto-optical storage medium which requires relatively compact and relatively light-weight components. In order to conserve space in the apparatus, the radiation sources and the magnets should all be positioned on the same side of the storage media. In addition, a need has been a magneto-optical medium upon which stores information on either side of the magneto-optical medium. Finally, a need has been felt for an apparatus and an associated method for simultaneously reading both sides of a magneto-optical medium upon which information has been written on both sides.